1. Field of the Invention:
This invention relates to methods and apparatus for the inspection of pipelines. More particularly, this invention relates to methods and apparatus for detecting and locating breaks in a non-conductive, protective coating of a subterranean pipeline by detecting electromagnetic fields generated by leakage current flowing through breaks in the protective coating.
2. Description of the Prior Art:
Electrically conductive metallic pipelines, particularly subterranean pipelines employed to convey crude oils, liquid petroleum products, slurried waxy hydrocarbons, natural gas, slurried coal particles, water and like materials are commonly wrapped and/or coated with a protective insulating material. The protective coating reduces or prevents exposure of the pipeline to the subterranean electrolytic environment and thereby reduces cathodic corrosion of the pipeline material. When a break in the protective coating (or a "holiday") occurs, the pipeline is subjected to electrolytic corrosion in the vicinity of the holiday, which if allowed to continue may result in product leakage and contamination of the surrounding environment and the product.
In order to avoid corrosive pipeline damage, various apparatus have been suggested for locating holidays in the protective coatings of pipelines. For example, U.S. Pat. No. 2,650,346 of Rasor discloses a device for applying a high test voltage across a protective coating between the pipeline and an electrical probe adapted to conform to and contact a portion of the outside of the coating to cause a spark to jump through holidays in the coating between the pipeline and the probe. U.S. Pat. No. 3,753,091 of Daspit discloses a device for detecting faults in non-conductive coatings of underwater pipelines as the pipelines are being laid by injecting an alternating current into the pipeline and sensing a magnetic field caused by leakage current with a sensing coil retained in close proximity to the pipeline by a support frame. These approaches, however, are applicable only to pre-installation inspection of a pipeline coating and cannot be used to detect holidays formed during installation or subsequently when the pipeline is buried in the ground or otherwise inaccessible.
Other apparatus have been suggested as being useful for detecting holidays in buried or submerged pipelines. For example, U.S. Pat. No. 3,066,256 of Rasor discloses locating flaws in the coating of a buried pipeline by applying an alternating current to the pipeline and moving a capacitive probe over the ground above the pipeline to detect an alternating field setup by leakage current, the probe producing an audible signal when the probe approaches a flaw in the coating. U.S. Pat. No. 2,501,598 of Eltenton et al. discloses a method of measuring the static magnetic field surrounding a pipeline by the use of a gradiometer. U.S. Pat. No. 3,745,452 of Osburn et al. discloses apparatus for detecting static direct current flowing at a holiday in a pipeline coating employing a vertically oriented flux gate magnetometer directly above the pipeline to sense the magnetic field associated with the holiday current. U.S. Pat. No. 3,536,831 of Smith teaches detecting flaws in the plastic coating of an undersea pipeline by applying an alternating current to the pipeline and detecting leakage current from the flaws with a pair of current conducting probes.